ARM: imx6q: add DT node for gpmi nand

Add the DT node for gpmi nand.
Add the pinmux support for gpmi nand.

The gpmi nand may conflicts with other modules, such as MMC.
So we do not enable the gpmi nand for mx6q-arm2 board, just add the
node for the board.

Acked-by: Dong Aisheng <dong.aisheng@linaro.org>
Signed-off-by: Huang Shijie <shijie8@gmail.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>